The invention relates to the handover method and a method for the selection of the base station for connection of a multimode terminal in a mobile communication system which consists of an MSC (Mobile Services Switching Center), at least one first base station and at least one second base station controlled by the MSC, and multimode terminals which consist of a first part for establishing a radio connection with the first base station and a second part for establishing a radio connection with the second base station.
The invention also relates to the multimode terminal.
FIG. 1a, attached, shows the block diagram of a mobile communication system the exchange of which controls the BSSs (Base Station Sub-systems) of the digital cordless telephone system DECT (Digital Enhanced Cordless Telecommunications) and the GSM (Global System for Mobile communications) mobile communication system. The MSC (Mobile Services Switching Centre) usually controls several base station systems BSS/FP. The MSC is connected to other MSCs and to other networks, such as the PSTN (Public Switched Telephone Network), PLMN (another mobile communication network), ISDN network, or IN (Intelligent Network). A multimode terminal that operates in several radio systems makes it possible to use several different radio systems for telecommunications traffic. The dualmode terminal PP/MS shown in FIG. 1a operates both as a cordless telephone of the DECT system (PP, Portable Part) and as an MS (Mobile Station) of the GSM system. The cordless telephone part of the dualmode terminal, PP, is connected in the DECT system to a DECT base station RFP (Radio Fixed Part), as shown in FIG. 1a, to the DECT base station RFP2. The fixed part of the base station system of the DECT system DECT FP (Fixed Part) consists of the base station controller CCFP (Common Control Fixed Part) which controls one or more DECT base stations RFP. The fixed part of the DECT system DECT FP is connected to the MSC via an adapter part IWU (Interworking Unit). The adapter part IWU performs, among other things, the necessary protocol conversions. In the GSM system the mobile station part MS of the dualmode terminal PP/MS is connected via radio path to a BTS (Base Transceiver Station). The BSS (Base Station System) consists of a BSC (Base Station Controller) and the base stations BTS controlled by it. The operation of the entire system is monitored by the OMC (Operation and Maintenance Centre).
In the DECT/GSM system it is also known that the functionality of the base station controller BSC and that of the DECT base station controller CCFP are connected into a single, so-called dualmode base station controller, as shown in FIG. 1b. This kind of a dualmode base station controller BSC/CCFP also includes the adapter part IWU. Otherwise the block diagram of FIG. 1b corresponds with FIG. 1a described above.
For use in call setup terminating in the terminal the registers of the cordless telephone system/mobile communication system contain the information about the geographic location of the terminal. In the GSM mobile communication system and the DECT cordless telephone system the coverage area of the network is divided into location areas which consist of the coverage areas of one or more base stations. The location of the terminal is defined by using these location areas. When the terminal has moved from one location area to another the terminal performs a location update to notify the network about its new location area. FIG. 2 shows the location update signalling for the DECT system. The cordless telephone PP sends the location update request to the base station to which it is listening by using the LOCATE REQUEST (message 21) to which is attached the identifier of the new location area. The fixed part of the network FP acknowledges the location update by using the LOCATE_ACCEPT (message 22), when the location area in the network register has been replaced with the stated new location area.
FIG. 3a attached, shows the call setup originating in the terminal PP in the DECT network. When the cordless telephone PP wants to make a call, it sends the CC_SETUP (message 31) to the base station RFP of the fixed part FP, which the network acknowledges by sending the CC_SETUP_ACK (message 32). If the cordless telephone PP has not indicated the number of the called subscriber to the network in the CC_SETUP (message 31), the telephone PP is requested for it in the CC_SETUP_ACK (message 32), in which case the terminal PP sends the number requested by using the CC_INFO (message 33). If the number of the called subscriber was indicated in the CC_SETUP (message 31), messages 32 and 33 are not sent at all. The traffic connection is established by using the CC_CALL_PROC (message 34). The CC_ALERTING (message 35) is used to send the line alert tone to the terminal PP. After this the call is continued in the normal fashion.
FIG. 3b attached, shows the call setup terminating in the terminal PP in the DECT network. The fixed part FP of the DECT network sends the call message LCE_REQUEST_PAGE (message 37) to reach the terminal PP. The cordless telephone PP answers this call by sending the LCE_PAGE_RESPONSE (message 38) after which the network notifies the terminal PP about the incoming call by sending the CC_SETUP (message 39). During the CC_ALERTING (message 40) the network is being sent the line alert tone, and after the subscriber has answered the terminal, the CC_CONNECT (message 41) is sent to establish the connection which the network acknowledges by sending the CC_CONNECT_ACK (message 42).
During the call, it may be necessary to hand over the radio connection between the terminal and base station from one radio channel to another, for example, because of a poor radio signal. Usually such a handover is performed from the serving base station to the radio channel of another base station. In cordless telephone systems, such as the DECT system, the handover process originates usually in the cordless terminal PP which measures the base station signals over the entire usable frequency band and selects a base station with a suitable radio signal as the target cell for the handover. In mobile communication systems, such as the GSM system, the handover originates usually in the side of the fixed network, for example, in the base station controller BSC.
FIG. 4 attached, shows a signaling diagram for the handover between two base station systems FP in the DECT system. Before starting the actual handover process, the terminal PP must receive the data about the possible target cells for the handover from the network. If the terminal PP has not received this data earlier, for example, during the call, the PP requests the data from the network by using the MM_INFO_REQUEST (message 43). The network transmits to the terminal PP via the serving base station, as shown in FIGS. 1a and 1b via the base station RFP2, in the MM_INFO_ACCEPT (message 44) the identifiers of those fixed parts FP into which the handover from the serving base station is possible. The terminal PP continuously measures the quality of the base station signal for these handover target cells and compares the measurement results to one another and to the quality of the signal of the serving base station RFP2. When the terminal PP detects that the signal quality of some other base station is better than the signal quality of the serving base station, it usually initiates the handover process. The terminal PP informs the serving base station system FP1 in the MM_INFO_REQUEST (message 45) about the initiation of the handover and about the target cell selected for the handover. When FP1 receives this information, it sends the HANDOVER_REQlIRED (message 46) to the mobile services switching center MSC, which requests the fixed part FP2 selected as the target cell for the handover to prepare for the handover by using the HANDOVER_REQUEST (message 47). The fixed part FP2 notifies the mobile services switching center MSC about the allocation of the resources of the target cell for the handover by using the HANDOVER_REQUEST ACK (message 48) after the reception of which the MSC notifies the base station system FP1 by using the HANDOVER_COMMAND (message 49) that the handover process can be continued. FP1 transmits this information by using the MM_INFO_ACCEPT (message 50) to the terminal PP, which sends the CC_SETUP (message 51) to the new base station system FP2. The CC_SETUP (message 51) includes the information that the process in question is the handover for an ongoing call instead of a new call setup. The FP2 sends information about the detection of the handover to the MSC by sending the HANDOVER_DETECT (message 52), after which the network connects the ongoing call to this new location. To notify the terminal PP about the acceptance of the handover, the fixed part FP2 sends the CC_CONNECT (message 53) to the terminal PP which the terminal PP acknowledges by sending the CC_CONNECT_ACK (message 54). The mobile services switching center MSC is notified of the success of the handover by using the HANDOVER_COMPLETE (message 55) after which the radio connection is used to convey information about the ciphering to be used in the connection and the former call connection established via the base station RFP2 is disconnected (not shown in the diagram).
The patent application publication GB-2 282 730 presents a handover method in a mobile communication system as shown in FIG. 1a to switch the radio connection of a dualmode terminal PP/MS from the radio channel of a DECT base station to a base station BTS of the GSM system. In the method described in the publication the mobile station part MS of the dualmode terminal continuously measures the signal level received from the base stations BTS of the GSM system and transmits the measurement results thus obtained to the cordless telephone part PP of the dualmode terminal PP/MS. When the radio connection on the cordless telephone side of the system gets so weak that it triggers the handover, the measurement results are transmitted from the cordless telephone PP further via the DECT base station RFP to the mobile services switching center MSC in the handover request. By using the received measurement results the MSC selects the base station BTS of the GSM system suitable for the handover into whose control the call of the dualmode terminal PP/MS is switched by using the handover.
The following problem occurs in the handover method presented in the GB publication. To determine the target cell for a possible handover, the mobile station part MS of the terminal must be continuously switched on to receive the GSM base station signals for measurements. This increases the power consumption of the terminal and thus consumes the limited battery capacity of the terminal thereby shortening the standby/operation time of the terminal.
The object of this invention is to recognize the base station of a mobile communication system whose radio signal is suitable to a multimode terminal by spending as little of the terminal power as possible, when it is desirable to switch the multimode terminal from a telecommunication system to use the services of a base station of another mobile communication system.
This is achieved by using a handover method according to the invention in a mobile communication system of the type presented in the introduction, the said method comprising the following steps: recognizing the poor radio connection between the serving second base station and the multimode terminal, selecting, as the target cell for the handover, one of the first base stations on the basis of the measurements performed in the multimode terminal, and performing the handover from the serving second base station to the selected first base station. It is characteristic to the handover method according to the invention that it further comprises the steps of: transmitting to the multimode terminal a cell description via the second base station, the said cell description containing the information about the frequency of at least one first base station located in the geographic area of the terminal, and performing a measurement of base station signals in the multimode terminal on frequencies indicated in the cell description, when the ongoing radio connection is detected to be poor.
The invention also relates to a method for selecting the base station for connecting the multimode terminal in a mobile communication system of the type presented in the introduction. It is characteristic for the method according to the invention that it comprises the steps of: transmitting to the multimode terminal via the second base station a cell description which contains the information about the frequency of at least one first base station located in the geographic area of the terminal, and selecting, as the base station to be listened to, one of the first base stations selected in the multimode terminal on the basis of the cell description, when it is desirable to switch to the service of one of the first base stations.
Furthermore, the invention relates to the multimode terminal which is adapted to operate in a mobile communication system, the said system consisting of at least one first base station and at least one second base station. It is characteristic for the multimode terminal according to the invention that it contains the equipment for storing the frequency data of at least one first base station transmitted from the second base station, and the equipment for selecting the base station to be listened to on the basis of the stored frequency data.
The invention is based on the idea that the multimode terminal connected to one telecommunication system is sent data about the frequency/frequencies used in another mobile communication system in the geographic area in question so that the multimode terminal can, if necessary, flexibly switch over from the one telecommunication system to be served by the mobile communication system. The local frequency data is transmitted to the multimode terminal preferably in the normal signaling of the telecommunication system attached to a message sent from the network to the terminal. When it is desirable to switch the multimode terminal to the service of the mobile communication system, the first part MS of the terminal synchronizes to the frequency indicated in the last received frequency data and, if necessary, connects to this base station.
The advantage of the method according to the invention is that the handover from the telecommunication system to the mobile communication system can be performed quickly and flexibly and with minimal power consumption on the part of the multimode terminal.
Another advantage of the method according to the invention is that the mobile station part of the terminal need not search the frequency of the base station signal of the mobile communication system, but it can directly tune itself to the cell broadcast frequency of the base station that broadcasts in the area.
Furthermore, another advantage of the method according to the invention is that the mobile station part of the multimode terminal can be kept in a power saving mode until the need for switching to the service of the mobile communication system is detected.